1. Field of the Invention
The present invention relates to particle detection in a manufacturing process, and in particular, relates to the control of particle sensor sensitivity parameters in response to environmental changes in the manufacturing process.
2. Discussion of the Related Art
Particle monitors are commonly used in semiconductor process equipment to detect the level of particles present, and to warn when such particle level exceeds an acceptable limit. FIG. 1 shows a typical configuration in a piece of semiconductor equipment which uses a particle monitor. As shown in FIG. 1, particle sensor 100 is positioned in the exhaust pump line 101 of process chamber 102 to detect particles carried in the exhaust gas flow. Typically, in a manufacturing process, the sources of particles are (i) process gasses admitted to the process chamber, (ii) the byproducts of the manufacturing process, or (iii) mechanisms in the chamber. A significant fraction of these particles present in process chamber 102 are carried down the exhaust line 101.
In addition, an exhaust line, such as exhaust line 101, typically has a throttle valve, such as throttle valve 103 shown in FIG. 1. The extent to which throttle valve 103 is open is typically controlled in accordance with the pressure required in every step of the manufacturing process. Typically, throttle valve 103 is controlled by a feedback loop involving a pressure gauge in process chamber 102. Throttle valve 103 is typically open between process runs to "base out" the chamber, i.e. to drop the pressure to the lowest possible value. After process chamber 102 is based out, throttle valve 103 is partially closed to raise the pressure in process chamber 102 to a pressure appropriate for the next process run.
Particle sensor 100 is typically a laser-light scattering sensor, which detects particles passing through a laser beam. An example of such a sensor is disclosed in U.S. Pat. No. 5,132,548, entitled "Large Detection Area Particle Sensor for Vacuum Applications" by P. Borden et al, filed on Sep. 14, 1990, assigned to High Yield Technology Inc., and issued on Jul. 21, 1992. In such a sensor, an electrical signal, in the form of a pulse, is generated whenever a detectable particle passes through the laser beam of the particle sensor. Because the laser beam of such a sensor has a fixed width, a faster particle generates a shorter signal pulse. This shorter pulse requires a high bandwidth in the particle sensor to ensure that detectable particles in the process chamber are fully detected. In fact, for practical purposes, the bandwidth requirement can be assumed to be directly proportional to the velocity of the particles to be detected. In addition, if particle sensor 100 is deployed in a plasma etcher, a plasma glow may be present in pump line 101. Such plasma glow provides optical noise which results in a reduced effectiveness in detecting smaller particles that scatter less light.
One disadvantage of using particle sensor 100 in a configuration, such as shown in FIG. 1, results from the fact that sensor operating parameters in such a configuration are fixed, and hence cannot be properly matched to critical parts of the process cycle, or to the requirements of the full process cycle. To achieve optimal performance, matching particle sensor 100's operating parameters to the changing process conditions is necessary. For example, when throttle valve 103 is open, the exhaust gas in pump line 101 achieves the full speed of the pump. As a result, the particles flowing through particle sensor 100 are travelling relatively fast. Under such a condition, a wide bandwidth is most appropriate for particle sensor 100.
However, a wide bandwidth particle sensor is more susceptible to noise, since the signal-to-noise ratio of a particle sensor degrades as the square root of the bandwidth. Thus, during the manufacturing process, when throttle valve 103 is partially closed and the particles in the exhaust gas passing through particle sensor 100's laser beam travel at a lower speed, a narrow bandwidth can be used to obtain an improved noise-immunity and hence achieve a better sensitivity.
As mentioned above, a sensor deployed in a plasma etcher can be exposed to a plasma glow in the pump line. For example, a nitrogen-oxygen mixture is used in certain photoresist stripping processes. This nitrogen-oxygen mixture generates a red flow that interferes with particle detection and may require de-tuning of the particle sensor. However, as the same plasma etcher is typically used in several processes, each of these several processes involving a different resist stripper, a plasma glow may be present in the plasma etcher for some processes, but not in others. Thus, the ability to match the sensitivity of the particle sensor to whether a plasma glow is present is desirable.
Today, all particle sensors have fixed sensitivity parameters, and cannot be adapted to one or more parts of the process cycle. Thus, it is desirable to dynamically control the particle sensor so that it may exhibit maximum sensitivity for all parts of the process.